全文获取类型
收费全文 | 709篇 |
免费 | 49篇 |
国内免费 | 14篇 |
专业分类
化学 | 490篇 |
晶体学 | 7篇 |
力学 | 29篇 |
数学 | 106篇 |
物理学 | 140篇 |
出版年
2023年 | 3篇 |
2022年 | 24篇 |
2021年 | 39篇 |
2020年 | 34篇 |
2019年 | 42篇 |
2018年 | 48篇 |
2017年 | 54篇 |
2016年 | 60篇 |
2015年 | 35篇 |
2014年 | 35篇 |
2013年 | 83篇 |
2012年 | 73篇 |
2011年 | 67篇 |
2010年 | 37篇 |
2009年 | 28篇 |
2008年 | 14篇 |
2007年 | 26篇 |
2006年 | 16篇 |
2005年 | 8篇 |
2004年 | 14篇 |
2003年 | 6篇 |
2002年 | 2篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1979年 | 2篇 |
排序方式: 共有772条查询结果,搜索用时 62 毫秒
751.
A simple and efficient liquid-phase microextraction (LPME) technique was developed using directly suspended organic microdrop coupled with gas chromatography–mass spectrometry (GC–MS), for the extraction and the determination of phthalate esters (dimethyl phthalate, diethyl phthalate, diallyl phthalate, di-n-butyl phthalate (DnBP), benzyl butyl phthalate (BBP), dicyclohexyl phthalate and di-2-ethylhexyl phthalate (DEHP)) in water samples. Microextraction efficiency factors, such as nature and volume of the organic solvent, temperature, salt effect, stirring rate and the extraction time were investigated and optimized. Under the optimized extraction conditions (extraction solvent: 1-dodecanol; extraction temperature: 60 °C; microdrop volume: 7 μL; stirring rate: 750 rpm, without salt addition and extraction time: 25 min), figures of merit of the proposed method were evaluated. The values of the detection limit were in the range of 0.02–0.05 μg L−1, while the R.S.D.% value for the analysis of 5.0 μg L−1 of the analytes was below 7.7% (n = 4). A good linearity (r2 ≥ 0.9940) and a broad linear range (0.05–100 μg L−1) were obtained. The method exhibited enrichment factor values ranging from 307 to 412. Finally, the designed method was successfully applied for the preconcentration and determination of the studied phthalate esters in different real water samples and satisfactory results were attained. 相似文献
752.
Prof. Hadi Khani Dr. Alain R. Puente Santiago Prof. Tianwei He 《Angewandte Chemie (International ed. in English)》2023,62(43):e202306103
The identity of alkali metal cations in the electrolyte of electrocatalysis systems has been recently introduced as a crucial factor to tailor the kinetics and Faradaic efficiency of many electrocatalytic reactions. In this Minireview, we have summarized the recent advances in the molecular-level understanding of cation effects on relevant electrocatalytic processes such as hydrogen evolution (HER), oxygen evolution (OER), and CO2 electroreduction (CO2RR) reactions. The discussion covers the effects of electrolyte cations on interfacial electric fields, structural organization of interfacial water molecules, blocking the catalytic active sites, stabilization or destabilization of intermediates, and interfacial pHs. These cation-induced interfacial phenomena have been reported to impact the performance (activity, selectivity, and stability) of electrochemical reactions collaboratively or independently. We describe that although there is almost a general agreement on the relationship between the size of alkali cations and the activities of HER, OER, and CO2RR, however, the mechanism by which the performance of these electrocatalytic reactions is influenced by alkali metal cations is still in debate. 相似文献
753.
Mukhtar MR Hadi AH Rondeau D Richomme P Litaudon M Mustafa MR Awang K 《Natural product research》2008,22(11):921-926
The phytochemical study of the bark of Malaysian Phoebe scortechinii (Lauraceae) has resulted in the isolation and identification of two new proaporphine alkaloids; (+)-scortechiniine A (1) and (+)-scortechiniine B (2) together with two known proaporphines; (-)-hexahydromecambrine A (3), (-)-norhexahydromecambrine A (4), and one aporphine; norboldine (5). Structural elucidations of these alkaloids were performed using spectroscopic methods especially 1D and 2D (1)H and (13)C NMR. 相似文献
754.
Iron(III) trifluoroacetate [Fe(CF3CO2)3] was found to be a recyclable, highly efficient and chemoselective Lewis acid catalyst for protection of a variety of carbonyl compounds as thioacetals under nearly neutral conditions. With the use of this catalyst, 1,3-dithiolanes and 1,3-dithianes were obtained in high yields from various aldehydes. Under the same conditions ketones were similarly but more slowly thioketalized. This difference in reactivity between aldehydes and ketones was successfully utilized for the selective thioacetalization of aldehydes in the presence of ketones and also for the chemoselective conversion of β-diketone into the corresponding dithioacetal. Transthioacetalization of O,O-acetals and O,O-ketals into cyclic thioacetals was also achieved by using this catalyst. Additionally, iron(III) trifluoroacetate has been found to be efficient catalyst for the addition of acetic anhydride to both aromatic and aliphatic aldehydes to afford 1,1-diacetates (gem diacetates). Aerobic dimerization of thiols was achieved by this reagent mediated by sodium iodide and air atmosphere. 相似文献
755.
Salmanzadeh A Romero L Shafiee H Gallo-Villanueva RC Stremler MA Cramer SD Davalos RV 《Lab on a chip》2012,12(1):182-189
In this study, the dielectrophoretic response of prostate tumor initiating cells (TICs) was investigated in a microfluidic system utilizing contactless dielectrophoresis (cDEP). The dielectrophoretic response of prostate TICs was observed to be distinctively different than that for non-TICs, enabling them to be sorted using cDEP. Culturing the sorted TICs generated spheroids, indicating that they were indeed initiating cells. This study presents the first marker-free TIC separation from non-TICs utilizing their electrical fingerprints through dielectrophoresis. 相似文献
756.
757.
Stengel F Baldwin AJ Bush MF Hilton GR Lioe H Basha E Jaya N Vierling E Benesch JL 《Chemistry & biology》2012,19(5):599-607
Small heat-shock proteins (sHSPs) are molecular chaperones that prevent irreversible aggregation through binding nonnative target proteins. Due to their heterogeneity, these sHSP:target complexes remain poorly understood. We present a nanoelectrospray mass spectrometry analysis algorithm for estimating the distribution of stoichiometries comprising a polydisperse ensemble of oligomers. We thus elucidate the organization of complexes formed between sHSPs and different target proteins. We find that binding is mass dependent, with the resultant complexes reflecting the native quaternary architecture of the target, indicating that protection happens early in the denaturation. Our data therefore explain the apparent paradox of how variable complex morphologies result from the generic mechanism of protection afforded by sHSPs. Our approach is applicable to a range of polydisperse proteins and provides a means for the automated and accurate interpretation of mass spectra derived from heterogeneous protein assemblies. 相似文献
758.
Zhang Z Xiang S Hong K Das MC Arman HD Garcia M Mondal JU Thomas KM Chen B 《Inorganic chemistry》2012,51(9):4947-4953
A three-dimensional triply interpenetrated mixed metal-organic framework, Zn(2)(BBA)(2)(CuPyen)·G(x) (M'MOF-20; BBA = biphenyl-4,4'-dicarboxylate; G = guest solvent molecules), of primitive cubic net was obtained through the solvothermal reaction of Zn(NO(3))(2), biphenyl-4,4'-dicarboxylic acid, and the salen precursor Cu(PyenH(2))(NO(3))(2) by a metallo-ligand approach. The triple framework interpenetration has stabilized the framework in which the activated M'MOF-20a displays type-I N(2) gas sorption behavior with a Langmuir surface area of 62 m(2) g(-1). The narrow pores of about 3.9 ? and the open metal sites on the pore surfaces within M'MOF-20a collaboratively induce its highly selective C(2)H(2)/CH(4) and CO(2)/CH(4) gas separation at ambient temperature. 相似文献
759.
Mohammad Behbahani Mohsen Taghizadeh Akbar Bagheri Hadi Hosseini Mani Salarian Atieh Tootoonchi 《Mikrochimica acta》2012,178(3-4):429-437
We describe a nanostructured ion-imprinted polymer (IIP) for the selective preconcentration of Ni(II) ions. It was obtained by bulk polymerization from 2-vinylpyridine (the functional monomer), ethylene glycol dimethacrylate (the cross-linker), 2,2′-azobisisobutyronitrile (the initiator), alizarin red S (the nickel-binding ligand), and nickel (the template ion) in acetonitrile solution. The IIP particles were characterized by elemental analysis, X-ray diffraction, Fourier transform IR spectroscopy, thermogravimetric and differential thermal analysis, and by scanning electron microscopy. Imprinted Ni(II) ions were removed from the polymeric structure using 5 % HCl as the eluting solvent. The material is capable of selectively binding Ni(II) from solutions at pH values between (pH 8.0 being best). Both the sorption and desorption process occur within 5 min. The maximum sorbent capacity of the ion imprinted polymer is 73 mg g?1. Following desorption, Ni(II) was determined by FAAS, with relative standard deviation and limit of detection of 3.4 % and 0.15 ng mL?1, respectively. The method was applied to the determination of nickel in certified reference materials (soil and polymetallic gold ore), fish, vegetables, river sediments, and river water. Figure
In this study, a novel nano structure Ni(II) ion imprinted polymer has been synthesized for faster extraction of Ni(II) ions from various matrices. This SPE technique was successfully applied for separation, determination, and preconcentration of nickel from food and environmental samples. This method is simple, rapid, and reliable and it is found to be a selective and sensitive method for determination of trace levels of Ni(II) ions. 相似文献
760.
Acute toxicity evaluation, antibacterial, antioxidant and immunomodulatory effects of Melastoma malabathricum 总被引:1,自引:0,他引:1
Alnajar ZA Abdulla MA Ali HM Alshawsh MA Hadi AH 《Molecules (Basel, Switzerland)》2012,17(3):3547-3559
Melastoma malabathricum (MM) is a well-known plant in Malaysian traditional medicine, locally known as senduduk. Its ethanol and aqueous extracts have been used in the present investigation to study the immunomodulatory role on human peripheral blood mononuclear cell (PBMC), and the DPPH, ABTS and FRAP free radical scavenging activities were also measured. Total flavonoids and total phenolic contents were assayed and the antibacterial effect was tested against four species of bacteria; two Gram-positive (Staphylococcus aureus and Streptococcus agalactiae) and two Gram-negative (Escherichia coli and Klebsilla pneumonia). The tests were carried out using the disc diffusion, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) methods. Moreover, the acute toxicity was evaluated in vivo on the ethanol extract of MM to establish its safety when administered orally. In our results, both extracts of MM showed abilities to scavenge DPPH and ABTS free radicals, IC(50) values: (11.599 ± 0.84, 10.573 ± 0.58 μmol/L) and (62.657 ± 0.78, 63.939 ± 0.48 μmol/L) for ethanol and aqueous extracts respectively. Indeed the ethanol extract evidenced high phenolic content (384.33 ± 0.005 mg/g), flavonoids contents (85.8 ± 0.009 mg/g) and ferric reducing antioxidant power (33,590 ± 0.038 mmol/g), with high activity against S. aureus and S. agalactiae (11 ± 0.3 and 12 ± 0.6 mm inhibition zones). Likewise, the percentage of peripheral blood mononuclear cells (PBMC) viability was increased in response to MM, IC(50) values (1.781 ± 1.2 and 6.545 ± 0.93 μg/mL) for ethanol and aqueous extracts, respectively. In addition, our results showed that the MM extract is safe even at a high dose of 5,000 mg/kg and has no oral toxicity. These findings suggest the excellent medicinal bioactivity of MM and explain the popularity of this plant in the folk medicine as a remedy for different illnesses. 相似文献